As known, the operating principle of the fiber optic gyroscope (FOG) is the Sagnac effect. When light transverses an optic fiber loop which is rotating about an axis perpendicular to its plane, the optical transit time of the light signal varies in dependence on the loop's rotation rate. For two optical signals transversing the loop in opposite directions the Sagnac phase difference between them (S; in radians) is proportional to the rotation rate, and is given by: ##EQU1## where: L is the length of the fiber loop, d is the loop diameter, .lambda. is the optical signal wavelength, c is the speed of light, and .OMEGA. is the loop rotation rate in radians/sec.
This difference, which serves as a measure of the rate, may be increased by using a fiber optic coil to increase the loop length (L). To alleviate the necessity of having to measure a DC value, a sinusoidal phase modulator may be used at one end of the loop to modulate the beams. The modulation acts on the counter circulating beams at different times due to the optical transit time delay in the coil, which results in a dithering of the phase difference. This improves the detection sensitivity by allowing the use of sensitive AC processing.
If opposing beams of unit intensity are added interferometrically the total intensity is: EQU I=1/2*(1+cos P) (Equation 2)
where P is total phase difference. Bessel expansion of the intensity expression produces a component: EQU F=k*sin(S) (Equation 3)
at the modulation frequency f. This can be taken as an analog output of the phase dithered Sagnac interferometer. The coefficient k is: EQU k=2*J.sub.1 *[2A*sin(.pi.*f*T)] (Equation 4)
where the term 2A*sin(.pi.*f*T) is the dither amplitude resulting from an applied modulation frequency (f) modulation of amplitude (A), with a coil transit time T. The amplitude is maximized when f=1/2T (the coil eigenfrequency).
The analog output F, of frequency f, is proportional to rotation at sufficiently small rates. It can be measured directly as an indication of rotation, or the output can be continuously nulled by a servo loop which adds an optical phase bias in opposition to the Sagnac phase difference. This can be achieved by adding a repetitive linearly ramped phase modulator (serrodyne modulation) at one end of the fiber coil. If the peak ramp amplitude is 2 .pi. radians, the serrodyne modulation produces an effectively constant phase difference bias between the oppositely directed beams. The phase bias amplitude is proportional to the ramp repetition frequency, which constitutes an easily measurable representation of the loop rotation rate.
FOG operation assumes the counter circulating beams travel identical paths in the absence of rotation and of applied phase bias, i.e. "reciprocity". However, different spatial modes and orthogonal polarization modes of the fiber coil are not always degenerate. Power coupled between modes can perturb the optical phase at the detector, causing drift in the FOG. Further, it may be difficult to simultaneously optimize phase modulation of the beam's desired and undesired polarization components, and gyro output error may result.
To eliminate cross coupling between spatial modes and to control polarization, prior art Sagnac gyros include both a single-spatial-mode filter and a single-polarizer filter in the beam propagation path. The filters are located in the optical path which is common to the source beam and the interfering beams; between the optical detector and the loop beam splitter. This "reciprocal configuration" confines the optical signals to a single spatial mode, and a single polarization. The polarizing filter may be a bulk optic polarizer, such as a Glan-Thompson prism which provides extinction ratios of 60 dB, or any other of a variety of fiber or integrated optic (IO) polarizers. The single mode filter may comprise a short length of single-mode fiber or waveguide.
However, unless the polarizing filter extinction ratio is sufficiently high, light polarized at the filter, which may become appreciably depolarized after traversing the loop, may arrive at the detector with significant power in the undesired polarization. In some cases the filter extinction ratio must be as high as 100 to 140 dB to reduce this power to acceptable levels.